Semiconductor devices, in particular visible light, light emitting diodes (LED) are used extensively for display devices and for various light sources. However, they have not yet been used as light emitting diodes in the ultra violet to blue light region. There has however been rapid development of light emitting diodes for use in displays requiring three basic colors. The expected ten fold increase in recording density by using laser diodes as a light source for optical disks and compact disks however has not yet been achieved. Light emitting diodes and lazer diodes emitting light in the ultra violet to blue color region use compound semiconductors such as GaN, ZnSe, ZnS, and SiC.
However, with these wide band gap compound semiconductors, it is generally difficult to produce single crystal thin films, and methods for manufacturing thin films which can be used for light emitting devices have yet to be established. For example, gallium nitride (GAN) which shows promise as a blue light emitting device, has up until now been grown as a thin film on the sapphire C (0001) face by means of the Metal Organic Chemical Vapor Deposition (MOCVD) method, or the Vapor Phase Epitaxy (VPE) method (Journal of Applied Physics 56 (1984) 2367-2368). With this method however it is necessary to have a high reaction temperature in order to obtain good crystallization. Consequently production is extremely difficult. Furthermore, since growth occurs at a high temperature, defects due to nitrogen deficiency can occur, and carrier density may become extremely large. As a result satisfactory semiconductor characteristics are difficult to obtain. In order to overcome these problems an aluminum nitride buffer layer is formed on the sapphire C (0001) face and a GaN thin film of a comparatively large film thickness is formed on top of this to make up a semiconductor light emitting elements.
In tests to achieve film growth at low temperatures, a method whereby the nitrogen supply gas is activated by irradiating it with an electron shower has been proposed (Japanese Journal of Applied Physics, 20, L545 (1981)). However, even with this method it is not possible to obtain film qualities suitable for light emission. Furthermore, to guard against nitrogen deficiency a highly activated nitrogen source is used in carrying out the film growth. To obtain the highly activated nitrogen a method utilizing plasma is used (Journal of Vacuum Science and Technology, A7, 701 (1989)). However up until now this method has been unsuccessful.
Investigations have also been made with GaInN mixed crystal thin films. Most of the tests involved thin film growth on the sapphire C face using the Metal Organic Vapor Phase Epitaxy (MOVPE) method (Journal of Applied Physics 28 (1989) L-1334). With this method however, the growth temperature for GaN and InN differs markedly making it difficult to obtain a good quality GaInN mixed crystal. Also, with GaAlN mixed crystals, an example of film growth by means of a gas-source molecular beam epitaxy (MBE) method using ammonia gas has been reported (Journal of Applied Physics 53 (1982) 6844-6848). However, with this method, although cathodoluminescence at liquid nitrogen temperatures was observed, a good quality thin film for the manufacture of light emitting devices was not obtained.
When using the conventional MOCVD and MOVPE methods for the manufacture of nitride semiconductor thin films, it is necessary to use a source material which contains carbon. Since the pressure during film growth is high, there are problems with the absorption of significant amounts of carbon impurities into the thin film so that a nitride semiconductor having poor qualities results.
Alternatively, there is a proposed construction wherein a single crystal thin film of Group III-V compound semiconductor including In and/or Ga is grown directly on a single crystal electrically insulating substrate (U.S. Pat. No. 4,404,265). With this proposal however, the following problems exist. The conditions required for directly growing the Group III-V compound semiconductor single crystal thin film on the substrate are extremely limited. Consequently the fabrication is not easy. Moreover, when growing a thin film of semiconductor directly on the substrate, a significant stress occurs in the semiconductor thin film due to the lattice mismatch between the substrate and the semiconductor, resulting in poor device durability. Furthermore, since the single crystal semiconductor is formed on the substrate, conductivity is reduced making it difficult to form a satisfactory ohmic contact for device operation.
With the nitride semiconductor thin films as described above, the GaAs semiconductor and Si semiconductor are different. Hence, since the semiconductor does not have a single crystal substrate of its own type, the thin film must be grown by the heteroepitaxy method. The production of thin films having good crystallization suitable for semiconductor devices, especially light emitting devices is thus extremely difficult.